The present invention relates to an inverter refrigerator which has a power circuit for rectifying alternating current to output a desired DC voltage and a motor control circuit for driving a motor.
Hitherto, a control unit for controlling the speed of a compressor motor by providing a rectifier circuit for rectifying AC to convert it into DC and by combining a power circuit, which suppresses higher harmonics of current and which controls the DC voltage, with a driving circuit for driving the compressor motor, has been disclosed in PCTJP 97/13318 (First Document).
The First Document describes a motor control circuit comprising a rectifier circuit and a smoothing circuit for converting AC power to DC, a converter circuit having a chopper circuit for controlling the DC voltage by utilizing an energy storage effect caused by switching operations and a reactor (inductance), a motor driving unit comprising an inverter circuit and a motor connected to the DC side of the converter circuit, an inverter control circuit for controlling the speed of the motor by controlling the switching operations of the inverter circuit, a speed detecting circuit for computing the speed of the motor by detecting the position of the rotor of the motor, a speed control circuit for controlling the speed of the motor via the inverter control circuit by taking in the computed value of speed and a value of a speed command, and a DC voltage control circuit for controlling the DC voltage via the converter control circuit by taking in an output signal of the speed control circuit and effecting control in accordance with the output signal.
The inverter control circuit drives the motor by driving a switching element of the inverter circuit to apply a rotating magnetic field to the motor based on a position signal from the speed detecting circuit and a conduction ratio signal from the speed control circuit. The speed detecting circuit detects the induced voltage of the motor to calculate the position of the rotor and outputs a pulse-like position detection signal. It also calculates the speed from the calculated position signal and outputs it to the speed control circuit as a speed detected value. Then, the speed control circuit calculates the conduction ratio signal of the PWM pulse of the inverter so that a deviation between the speed command from the outside and the speed detected value is zeroed. The speed of the motor is controlled by the inverter circuit, the motor, the speed detecting circuit, the inverter control circuit and the speed control circuit described above.
The converter control circuit drives the switching element of the chopper circuit in accordance with the signal from the DC voltage control circuit. The DC voltage control circuit detects the DC voltage and the output signal of the speed control circuit, e.g., the conduction ratio signal, and controls the DC voltage so as to raise the DC voltage by a predetermined width when the conduction ratio signal reaches a predetermined value, e.g., at the upper limit within a certain range of the conduction ratio, or controls the DC voltage so as to drop the DC voltage by a predetermined width when the conduction ratio signal reaches the lower limit value. The DC voltage control circuit of the converter is formed by the converter circuit, the converter control circuit and the DC voltage control circuit and operates to control the DC voltage.
Although the motor control unit described in the First Document has not been described with regard to possible use for a refrigerator, one using so-called PAM control means for controlling a DC voltage as a motor control unit for driving a refrigerator compressor has been described in JP-A-7-260309 (Second Document) and JP-A-7-218097 (Third Document).
Although the Second and Third Documents have suggested that energy may be saved by using the PAM inverter as a controller of a motor for driving a compressor of a refrigerator, they have provided so specific proposal for saving energy while performing those functions required by a refrigerator. The structure described in the First Document has not been considered for use in a refrigerator, so that it provides no disclosure concerning energy saving. U
The power voltage (AC voltage supplied to house-hold plugs) for driving the motor becomes .+-.7.5% of the reference value when an allowable variation prescribed by the electric utility law and a voltage drop within a home are taken into consideration. The conventional controller of the motor using a voltage doubling circuit has a difference of voltage in the DC stage of 43 V between the maximum value and the minimum value of 260 V to 303 V, so that there is a situation in which the motor is not activated when the voltage of the DC stage is low.